I have no idea how easy this would be to do, but there aren't a lot of programs that would allow you to view the graphics you're making in a custom aspect ratio.

So, you may be asking yourself "why would I want to view my graphics in 9:8?", and the short answer is that on the NES hardware, it tends to not display square pixels. The pixels are stretched horizontally just a bit, making graphics slightly wider than they would appear on your computer screen. This is why NES games on emulators look a bit off, or a little too skinny. It's not immediately noticeable, but once you know about it, it's very difficult to un-see.

A 9:8 aspect ratio (basically making the representation of each NES pixel to appear as a block that's 8 pixels tall and 9 pixels wide on your computer screen) would much more closely represent the graphics as they'll probably appear on your NES machine. Working with 1:1 square pixels isn't terrible, you can always just keep in mind that it's going to be stretched a bit, but having this as a feature next to the "force square pixels" button would be awesome.